1. Field of the Invention
The present system relates to improvements to a step-by-step remote locating system for repeater pairs in a PCM link.
2. Description of the Prior Art
More particularly, the invention concerns a system for step-by-step remote locating pairs of first and second repeaters included in a bidirectional digital transmission medium between first and second end equipments. Such a remote locating system is disclosed in U.S. Pat. No. 4,402,075.
In this system, the transmission medium is a link comprising a first transmision channel in which the first repeaters are interconnected via first respective switches which are closed thereby transmitting a first digital signal from the first equipment to the second equipment, and a second channel in which the second repeaters are interconnected via second respective switches which are closed thereby transmitting a second digital signal from the second equipment to the first equipment.
The first equipment comprises a means including a remote locate sequence generator and a central remote control unit for chopping the first digital signal with interruption sequences. Each of the sequences comprises k first interruptions having a predetermined width and a last interruption preceded respectively by k first signal fields having a predetermined width and a last field k, where k is an integer. r sequences are transmitted consecutively in the first channel to remote locate a pair of repeaters defined by an integer rank r counted from the first equipment. The last field of the last of the r sequences has a width greater than that of the other signal fields.
Each pair of first and second repeaters is associated with a means including a timing circuit and a counter-decoder for counting the interruptions in each sequence and with a means including a retriggerable monostable flip-flop for resetting the counting means in response to an interruption in the first signal having a first width exceeding the sum of the widths of a first field and a first interruption in a sequence. The counting means commands the opening of the respective first switch thereby disconnecting the first respective repeater from the first repeater of following rank, and an opening of the second respective switch thereby disconnecting the second respective repeater from the second repeater of following rank and connecting said first and second respective repeaters of said pair through a respective loop-back path as soon as the counting means has reached a total equal to k. The counting means also commands closure of the respective first and second switches when the count is other than k.
In this system, when a pair of repeaters is remote located, which results in a link via the loop-back path between the repeaters of this pair for transmission of a digital test slignal on the second channel to the first end equipmen, said test signal being initially transmitted by this first equipment on the first channel, the first respective switch means is open. In other words, the first repeater of said pair is disconnected from the first following repeaters of higher rank next to the second equipment, throughout the duration of the remote located pair test.
As is known, each repeater comprises a self-matching equalizer including a variable gain amplifier, called as a channel section corrector, and an automatic gain control circuit (AGC) for compensating the attenuation of the digital signal received via the respective channel section. Given that the last signal field in the last remote location sequence contains a digital test signal which is intended to be analyzed by the first equipment in order to detect faults in the located repeater pair and that this latter field has a relatively long duration, the gain control circuits in the first repeaters disconnected from the first end equipment react as if the first channel sections preceding them were extremely long, greater than the maximum length of the channel sections. In this case, the amplification gain of the equalizers in the disconnected repeaters is maximum. This same maximum gain phenomenon occurs in the system described in U.S. Pat. No. 4,402,075, when one of the repeater pairs preceding the pair to be remote located is momentarily looped, the width of the last sequence interruption being considerably greater than the width of the other interruptions of the sequence.
It so happens that disconnection of the first repeaters from the first end equipment by opening of the respective first switches or by opening of the first switch of a remote located pair, can in practice cause untimely looping of the disconnected repeater pairs owing to a noise signal, and/or trigger a regeneration fault in the remote location sequence transmitted after unlooping of a previous pair, because of the maximum gain in the gain control circuit which does not allow suitable regeneration and therefore correctrepeating of a remote locate sequence.